Conventionally, fastening members having thread structures such as a bolt and a nut have been used widely. In order to actually enable fastening the bolt to the nut, it is necessary to provide dimensional tolerance in an outer diameter and an effective diameter of the bolt and an inner diameter and an effective diameter of the nut. However, there is a possibility that the tolerance causes loosening. Conventionally, various devices for inhibiting the occurrence of the loosening have been made.
Patent document 1 discloses a tightening screw structured as follows. That is, a flat flank portion on a thread tip end side and a pressure-contact flank portion continuous to the flat flank portion are formed on a pressure-contact flank, which is one of both flanks of a thread of an external screw or an internal screw and which serves as a pressure-contact side in a fastened state. An inclination of the pressure-contact flank portion is formed to be larger than an inclination of a flank of an internal screw or an external screw facing the pressure-contact flank portion.
Patent document 2 discloses an external screw, in which a thread external diameter is formed to be larger than a reference value and a half angle on a top side of the thread is set at an angle smaller than 30 degrees.
There is also a method to inhibit the loosening by forming a resin coating layer on a part of a thread.
Further, Patent documents 3 to 7 may be also referred to as prior literatures disclosing technologies related to the present invention.